axfr performs a DNS zone transfer,
prints each record to the standard output,
and stores the zone to a file.
If the zone has already been stored in a file,
axfr will read the file instead of performing a zone transfer.

Zones will be stored in a directory hierarchy.
For example,
the zone transfer for foo.bar.com will be stored in the file $HOME/.dns-zones/com/bar/foo/axfr.
The directory can be changed with the -D option.